A Case Study
J. A. R. Pacheco de Carvalho
, N. Marques
, H. Veiga
, C. F. Ribeiro Pacheco
U. de Detecção Remota,
Dept. de Física,
Centro de Informática, Universidade da Beira Interior
6201-001 Covilhã, Portugal
A. D. Reis
Dept. de Electrónica e Telecomunicações, Instituto de Telecomunicações
Universidade de Aveiro, 3810 Aveiro, Portugal
Keywords: Wireless Network, Laser, FSO, Point-to-Point Link, Performance Measurements.
Abstract: Wireless communications have been increasingly important. Besides Wi-Fi, FSO plays a very relevant
technological role in this context. Performance is essential, resulting in more reliable and efficient
communications. A FSO medium range link has been successfully implemented for high requirement
applications at Gbps. An experimental performance evaluation of this link has been carried out at OSI layers
1, 4 and 7, through a specifically planned field test arrangement. Several results are presented and discussed,
as obtained from simultaneous measurements of powers received by the laser heads, TCP, UDP and FTP
experiments, resulting in determinations of TCP throughput, jitter, percentage datagram loss and FTP
transfer rate. Conclusions are drawn about link performance.
Wi-Fi and FSO are wireless communications
technologies whose importance and utilization have
been growing.
Wi-Fi uses microwaves in the 2.4 and 5 GHz
frequency bands and IEEE 802, 11a, b, g standards.
Nominal transfer rates up to 11 (802.11b) and 54
Mbps (802.11a, g) are specified (IEEE Std 802.11-
2007). It has ben used in ad hoc and infrastructure
modes. Point-to-point and point-to-multipoint
configurations are used both indoors and outdoors,
requiring specific directional and omnidirectional
FSO uses laser technology to provide
point-to-point communications e.g. to interconnect
LANs of two buildings having line-of-site. FSO was
developed in the 1960’s for military and other
purposes, including high requirement applications.
At present, speeds typically up to 2.5 Gbps are
possible and ranges up to a few km, depending on
technology and atmospheric conditions. Interfaces
such as fast Ethernet and Gigabit Ethernet are used
to communicate with LAN’s. Typical laser
wavelengths of 785 nm, 850 nm and 1550 nm are
used. In a FSO link the transmitters deliver high
power light which, after travelling through
atmosphere, appears as low power light at the
receiver. The link margin of the connection
represents the amount of light received by a terminal
over the minimum value required to keep the link
active: (link margin)
= 10 log
), where P
and P
are the corresponding power values,
There are several factors related to performance
degradation in the design of a FSO link: distance
between optical emitters; line of sight; alignment of
optical emitters; stability of the mounting points;
atmospheric conditions; water vapour or hot air;
strong electromagnetic interference; wavelength of
the laser light (Rockwell & Mecherle, 2001). A
redundant microwave link is always essential, as the
laser link can fail under adverse conditions. Several
studies and implementations of FSO have been
reported (D’Amico, Leva & Micheli, 2003). FSO
has been used in hybrid systems for temporary
multimedia applications (Mandl et al., 2007).
A. R. Pacheco de Carvalho J., Marques N., Veiga H., F. Ribeiro Pacheco C. and D. Reis A. (2010).
In Proceedings of the International Conference on Wireless Information Networks and Systems, pages 123-128
DOI: 10.5220/0002978601230128
Performance has been a very important issue,
resulting in more reliable and efficient
communications. Telematic applications have
specific performance requirements, depending on
application. New telematic applications present
special sensitivities to performances, when
compared to traditional applications. E.g.
requirements have been quoted as: for video on
demand/moving images, 1-10 ms jitter and 1-10
Mbps throughput; for Hi Fi stereo audio, jitter less
than 1 ms and 0.1-1 Mbps throughputs (Monteiro e
Boavida, 2002).
Several performance measurements have been
made for 2.4 and 5 GHz Wi-Fi (Pacheco de
Carvalho et al., 2008a, 2009). FSO and fiber optics
have been applied at the University Campus to
improve communications quality (Pacheco de
Carvalho et al., 2007, 2008b, 2008c). In the present
work we have further investigated that FSO link for
performance evaluation at OSI layers 1, 4 and 7.
The rest of the paper is structured as follows:
Chapter 2 presents the experimental details i.e. the
measurement setup and procedure. Results and
discussion are presented in Chapter 3. Conclusions
are drawn in Chapter 4.
The main experimental details, for testing the quality
of the FSO link, are as follows.
A 1 Gbps full-duplex link was planned and
implemented, to interconnect the LAN at the Faculty
of Medicine building and the main University
network, to support medical imaging, VoIP, audio
and video traffics (Pacheco de Carvalho et al., 2007,
2008b). Then, a FSO laser link at 1 Gbps full-
duplex, over a distance of 1.14 km, was created to
interconnect the Faculty of Medicine (FM) building
at Pole III and the Sports (SB) building at Pole II of
the University (Figure 1).
We have chosen laser heads from FSONA
(Figure 2) to implement the laser link at a laser
wavelength of λ= 1550 nm for eye safety, where
allowable laser power is about fifty times higher at
1550 nm than at 800 nm (Rockwell & Mecherle,
2001). Each laser head comprised two independent
transmitters, for redundancy, and one wide aperture
receiver. Each laser had 140 mW of power, resulting
in an output power of 280 mW (24.5 dBm). 1000-
Base-LX links over OM3 50/125 μm fiber were used
to connect the laser heads to the LANs.
For a matter of redundancy a 802.16d WiMAX
point-to-point link at 5.4 GHz (IEEE Std 802.16-
2004) was available, where data rates up to either 75
Mbps or 108 Mbps were possible in normal mode or
in turbo mode, respectively (Alvarion, 2007). This
link was used as a backup link for FM-SB
communications, through configuration of two static
routing entries in the switching/routing equipment
(Pacheco de Carvalho et al., 2007).
Performance tests of the FSO link were made
under favourable weather conditions. During the
tests we used a data rate mode for the laser heads
which was compatible with Gigabit Ethernet. At OSI
layer 1 (physical layer), received powers were
simultaneously measured for both laser heads. Data
were collected from the internal logs of the laser
heads, using STC (SONAbeam Terminal Controller)
management software (FSONA, 2006). At OSI layer
4 (transport layer), measurements were made for
TCP connections and UDP communications using
Iperf software (NLANR, 2005), permitting network
performance results to be recorded. . Both TCP and
UDP are transport protocols. TCP is connection-
oriented. UDP is connectionless, as it sends data
without ever establishing a connection. For a TCP
connection over a link, TCP throughput was
obtained. For a UDP communication, we obtained
UDP throughput, jitter and percentage loss of
datagrams. TCP packets and UDP datagrams of
1470 bytes size were used. A window size of 8
kbytes and a buffer size of the same value were used
for TCP and UDP, respectively.
A specific field test arrangement was planned
and implemented for the measurements (Figure 3).
Two PC’s having IP addresses and were setup as the Iperf server and client,
respectively. The PCs were HP computers, with 3.0
GHz Pentium IV CPUs, running Windows XP. The
server had a better RAM configuration than the
client. They were both equipped with 1000Base-T
network adapters. Each PC was connected via
1000Base-T to a C2 Enterasys switch (Enterasys,
2008). Each switch had a 1000Base-LX interface.
Each interface was intended to establish a FSO link
through two laser heads, as represented in Figure 3.
The laser heads were located at Pole II and Pole III,
at the SB and FM buildings, respectively. The
experimental arrangement could be remotely
accessed through the FM LAN. In the UDP tests a
bandwidth parameter of 300 Mbps was used in the
Iperf client. Jitter, which represents the smooth mean
of differences between consecutive transit times,
was continuously computed by the server, as
specified by RTP in RFC 1889. RTP provides end-
to-end network transport functions appropriate for
applications transmitting real-time data, e.g. audio,
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video, over multicast or unicast network services. At
OSI layer 7 (application layer) the setup given in
Figure 3 was also used for measurements of FTP
transfer rates through FTP server and client
applications installed in the PCs. Each measurement
corresponded to a single FTP transfer, using a 2.71
Gbyte file. Whenever a measurement was made at
either OSI layer 4 or 7, data were simultaneously
collected at OSI layer 1. Batch command files were
written to enable the TCP, UDP and FTP tests. The
results, obtained in batch mode, were recorded as
data files in the client PC disk.
Large amounts of data were collected and processed
by averaging over several time intervals. The
corresponding results are shown for TCP in Figure
4, for UDP in Figure 6 and FTP in Figure 8. The
average received powers for the SB and FM laser
heads, mostly ranged high values in the 25-35 µW
interval which corresponds to link margins of 4.9-
6.4 dB (considering P
=8 µW). From Figure 4 it
follows that TCP average throughput (313.6 Mbps)
is reasonably steady, although some small peaks
arise for throughput deviation. Figure 5 illustrates
details of TCP results over a small interval. Figure 6
shows that UDP average throughput (125.5 Mbps) is
fairly steady, having a small steady throughput
deviation. The jitter is small, usually less than 1 ms,
while percentage datagram loss is practically
negligible. Figure 7 illustrates details of UDP-jitter
results over a small interval. Figure 8 shows that
average FTP throughput (344.5 Mbps) is very
steady, having low throughput deviation. Figure 9
illustrates details of FTP results over a small
interval. Transfer rates of the PC’s disks are always
a limitation in this type of FTP experiments. In all
cases, having high values of average received
powers, the quantities under analysis did not show
on average significant variations even when the
received powers varied. The results here obtained
complement previous work by the authors (Pacheco
de Carvalho et al., 2007, 2008b, 2008c). Generally,
for our experimental conditions, the FSO link has
exhibited very good performances at OSI layers 4
and 7.
Besides the present results, it must be mentioned
that we have implemented a VoIP solution based on
Cisco Call Manager (Cisco, 2004). VoIP, with
G.711 and G729A coding algorithms, has been
working over the laser link without any performance
problems. Tools such as Cisco IP Communicator
have been used. Video and sound have also been
tested through the laser link, by using eyeBeam
Softphone CounterPath software (CounterPath,
2007). Applications using the link have been well-
Figure 1: View of the 1.14 km laser link between Pole II
(SB) and Pole III (FM).
Figure 2: View of the laser heads at FM (Pole III) and SB
(Pole II).
Figure 3: Field tests setup scheme for the FSO link.
Figure 4: TCP results.
Figure 5: Details of TCP results.
Figure 6: UDP results; 300 Mbps bandwidth parameter.
Figure 7: Details of UDP-jitter results; 300 Mbps
bandwidth parameter.
Figure 8: FTP results.
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Figure 9: Details of FTP results.
An FSO laser link at 1 Gbps has been successfully
implemented over 1.14 km along the city, for
interconnecting Poles of the University and support
high requirement applications.
A field test arrangement was planned and
implemented for FSO performance measurements at
OSI layers 1, 4 and 7. At OSI layer 1, received
powers were simultaneously measured in both laser
heads. At OSI layer 4, TCP throughput, jitter and
percentage datagram loss were measured. Through
OSI layer 7, FTP transfer rate data were acquired.
Under favourable experimental weather conditions,
when the measurements were carried out, the link
has shown to be very well behaved, presenting very
good performances. Applications such as VoIP,
video and sound, have been well-behaved. Further
measurements are planned under several
experimental conditions, such as environmental and
multimedia traffic.
Supports from the University of Beira Interior and
FCT (Fundação para a Ciência e a
Tecnologia)/POCI2010 (Programa Operacional
Ciência e Inovação) are acknowledged. We
acknowledge Hewlett Packard and FSONA for their
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